Recently, an attention has been focused on small-turn type hydraulic excavators wherein a work front is designed to be able to turn with a small radius so that the excavator can work in a narrow place. It is essential for these small-turn type hydraulic excavators to have a function of enabling a side trench to be dug in a position near and transversely of an excavator body without changing the body position. To this end, hydraulic excavators of the offset and swing types have been proposed. There are known the following examples (1) an example of the offset type and (2), (3) examples of the swing type.
(1) JP, A, 3-255241
According to this known art, an arm is made up of an offset arm coupled to a front end of a boom in such a manner as to be able to be offset to the left and right, and a bucket arm capable of pivotally moving up and down with respect to the offset arm while being kept parallel to the boom. This known art also discloses a structure in which a longitudinal groove is formed in an upper cover, which covers most of an area over a turning frame except for the cab, to prevent interference between the boom and the upper cover when the boom is maximally lifted upward.
(2) JP, A, 5-125743
This known art discloses a swing type hydraulic excavator wherein a boom is swingable to the left and right about a swing post in a position on the front end side of a turning structure.
(3) JP, A, 7-243223
This known art discloses a structure that, in a swing type hydraulic excavator similar to that of the above (2), includes a boom having a two-piece boom made up of a lower boom capable of pivotally moving up and down with respect to a swing post and an upper boom capable of pivotally moving up and down with respect to the lower boom, and an opening angle adjusting means for changing an angle between the lower boom and the upper boom depending on the angle of pivotal movement of the lower boom.